1. Field of the Invention
The present invention relates to an image forming apparatus provided with a charging section employing a proximity electrification system and receiving a charged voltage which includes a direct-current voltage and an alternating-current voltage superimposed thereon.
2. Description of Related Art
In recent years, in view of, for example, ozone depletion, the proximity electrification system is becoming a mainstream charging system for image forming apparatuses. In the case of the proximity electrification system, a charging section in the form of, for example, a roller, a brush, or a blade is disposed in proximity to and either in or out of contact with the surface of a photoreceptor drum, which is a typical example of an image carrier.
In a predetermined process such as a printing process, the charging section receives a charged voltage which includes a direct-current voltage and an alternating-current voltage superimposed thereon and having a predetermined peak-to-peak voltage value Vpp1, so that the surface of the image carrier is uniformly charged (see, for example, Japanese Laid-Open Patent Publication No. 2001-201920).
In Japanese Laid-Open Patent Publication No. 2001-201920, to allow stable discharge between the charging section and the image carrier, the charging section is sequentially provided by a power supply with alternating-current voltages having different detection peak-to-peak voltage values Vpp0 both in a forward discharge range, where only a charge transfer from the charging section to the image carrier occurs (i.e., unidirectional charge transfer), and in a back discharge range, where a bidirectional charge transfer between the charging section and the image carrier occurs in an alternating manner. For the forward discharge range and the back discharge range, a control section derives approximation functions f1(Vpp) and f2(Vpp) of alternating-current value Iac to peak-to-peak voltage value Vpp, and thereafter, derives the predetermined peak-to-peak voltage value Vpp1 and a discharge current amount ΔIac, the difference function (ΔIac (=f2(Vpp)−f1(Vpp)) of which takes a predetermined value D.
In Japanese Laid-Open Patent Publication No. 2001-201920, the predetermined value D is constant regardless of the degree of wear and tear on the image carrier due to repeated printing and other factors. However, the inventors of the present invention found out through experimentation that an optimal predetermined value D varies depending on the degree of wear and tear. More specifically, the optimal predetermined value D decreases as wear and tear progresses. Accordingly, the peak-to-peak voltage value Vpp1, which is optimal at the initial stage of the life of the image carrier, is unnecessarily high (i.e., not optimal) at the last stage of life, so that the image carrier is damaged significantly. As a result, wear and tear is accelerated, so that the life of the image carrier becomes shorter than designed.